1. Field
The present disclosure relates to a method for stopping a fuel cell system and a fuel cell system.
2. Description of the Related Art
Examples of a fuel cell unit in a fuel cell system mounted on a fuel cell vehicle include a fuel cell stack formed by stacking a plurality of cells. Each of the cells includes an anode, a cathode, a solid polymer electrolyte membrane, such as a solid polymer ion-exchange membrane, interposed between the anode and the cathode, and a pair of separators disposed on respective outer sides of the anode and the cathode.
The fuel cell unit generates electric power by using, as a fuel gas, a hydrogen gas supplied through a fuel supply passage to the anode of each cell and also using, as an oxidant gas, air containing oxygen supplied through an oxidant supply passage to the cathode of each cell. The fuel supply passage is provided with a hydrogen shutoff valve that permits or shuts off the supply of hydrogen gas, and the oxidant supply passage is provided with an air shutoff valve that permits or shuts off the supply of air.
In the fuel cell unit configured as described above, hydrogen ions generated by catalysis at the anode travel through the solid polymer electrolyte membrane to the cathode, where the hydrogen ions electrochemically react with oxygen to generate electric power.
It is known that a so-called crossover occurs in a fuel cell unit of this type. The crossover is a phenomenon in which, in the process of stopping power generation, the hydrogen gas remaining on the anode side in the fuel cell unit passes through the solid polymer electrolyte membrane and diffuses to the cathode side, whereas oxygen and nitrogen gases in the air on the cathode side pass through the solid polymer electrolyte membrane and diffuse to the anode side. The crossover may cause an electrochemical reaction between reaction gases near the solid polymer electrolyte membrane, and this may deteriorate the solid polymer electrolyte membrane.
A technique is disclosed as a solution to this. In this technique, in the process of stopping power generation of a fuel cell unit, an air off-gas discharged from a cathode of the fuel cell unit is recirculated and supplied to the cathode, so as to continue the power generation using residual oxygen in the air off-gas. The power generation is stopped when a power generation voltage becomes lower than a predetermined value.
During normal power generation or during power generation using residual oxygen in air off-gas, a hydrogen shutoff valve and an air shutoff valve are open. These shutoff valves are closed at the end of the process of stopping the power generation. This can prevent additional hydrogen from entering an anode side of the fuel cell unit, and can also prevent additional air from entering a cathode side of the fuel cell unit. Thus, since the cathode side can be kept in a nitrogen-rich state where the oxygen concentration is very low, it is possible to reduce damage to a solid polymer electrolyte membrane (see, e.g., Japanese Unexamined Patent Application Publication Nos. 8-195210 and 2013-149538).